Corals - a significant source of crucial atmospheric gas at low tide

Plymouth Marine Laboratory-led research shows that tropical corals at low tide emit the important climate gas dimethylsulfide directly to the atmosphere in amounts equal to other, better known, sources.

They conclude that this source, which has so far been overlooked by climate models, may be of key importance to the tropical atmosphere – and now requires further measurements in the field.

Corals are prolific producers of dimethylsulfonioproprionate (DMSP) which breaks down into an important atmospheric gas, dimethylsulfide (DMS). DMS is a major source of sulfur to the atmosphere, providing a key step in cloud formation, which in turn forms a part of the climate regulation of the Earth. Although DMSP is produced by the corals themselves, most production is due to the symbiotic algae within their tissues that give corals their bright colours. DMSP is a useful compound for corals and their algae partners, protecting them from cell damage. It is readily converted to DMS which continually seeps into the surrounding water and while much of this is further broken down before it reaches the sea surface, quantities may directly reach the atmosphere when corals are exposed to air. Previous research has shown that DMS concentrations in the atmosphere above coral reefs rise when the corals are exposed at low tide. Now experiments and measurements carried out by scientists at Plymouth Marine Laboratory (PML), along with colleagues at the University of Essex, UK and the University of Technology Sydney, Australia, reveal the processes that are responsible.

Experiments on three species of tropical corals included submergence, simulating low tide and exposure to air by removing the seawater surrounding the corals, followed by re-submergence. The gas phase concentrations of DMS rose by an order of magnitude at simulated ‘low tide’, when corals inhabiting reef flats and crests may be exposed for long periods suffering temperature rise, increased sunlight levels and desiccation. As the corals were re-submerged, simulating a returning tide, there was a further pulse of DMS. This may reflect increased production of DMS as a reaction to the stress of exposure and submergence when harmful radicals are released into cells, or perhaps dissolution of the DMS-rich mucous formed as a protective layer by the corals on exposure to air.

It was, however, the amounts of DMS that really caught the attention of the scientists, PML biogeochemist Dr Frances Hopkins explains: “Our experiments suggest that DMS emissions from coral reefs may be in the same order of magnitude as other marine DMS hotspots. Blooms of plankton, such as coccolithophores and dinoflagellates, are perhaps better known as DMS sources but the concentration above these blooms may be up to one part/billion (1 ppb); above coral reefs it can be 19 ppb or more! Another significant difference is that whereas these other sources tend to be transitory, seasonal or life cycle driven for example, coral reefs are a more consistent source of DMS.”

Dr Michael Steinke from University of Essex added: “We recognise that each reef is different and that our experiments were limited, but the indications are that this largely unquantified source of DMS is a significant contributor to the tropical atmosphere. This large input of DMS from coral reefs is currently unrecognised in climate models so we now need in situ measurements of DMS above reefs to really get a handle on how much they may contribute to global DMS fluxes. Bearing in mind how DMS can influence cloud formation and climate, any changes such as coral bleaching, which results from the expulsion of the coral’s symbiotic algae, could have implications for local atmospheric DMS fluxes.”